Maternally-inherited, facultative bacterial endosymbionts of insects have been shown to manipulate host reproduction, protect hosts against pathogens or parasitoids, and change the plant host range of some herbivores, but the roles of the many facultative bacterial symbionts of the cosmopolitan whitefly pest, Bemisia tabaci, are largely unknown. We have been studying one of these, Rickettsia sp. nr. bellii, in the B. tabaci present in Arizona. We have evidence of a dramatic sweep of Rickettsia infection in the Southwestern United States from 1% to 97% over six years. How did it spread? Experiments suggested that some parasitoids are able to pick up the Rickettsia infection from infected whiteflies, but we find little evidence of horizontal transmission from infected whiteflies to uninfected whiteflies sharing the same plant. Theory predicts that a strictly vertically transmitted symbiont must manipulate host reproduction or confer fitness benefits. We investigated the effects of Rickettsia on host phenotype using introgressed whitefly lines that differed by the presence/absence of Rickettsia.
Results/Conclusions
Rickettsia appears to confer a dramatic fitness benefit and skew whitefly sex ratios towards females, both of which likely contribute to spread. Rickettsia-infected individuals have much greater developmental success, appear to develop faster, and lay more eggs. The offspring sex ratio of Rickettsia-infected females is also double the proportion of females relative to those without Rickettsia. In population cage experiments in the laboratory, we show rapid spread of the Rickettsia infection on three host plants: cotton, cowpea, and melons. Over five generations, the percentage of infected individuals increased from the starting frequency of 15% to 56%, suggesting that the agents of selection causing spread in the field are also present in the laboratory environment. Current work seeks to identify the mechanism by which Rickettsia increases whitefly fitness.